I am totally with John Woodgate on this 20% deltas are insignificant in the larger picture. I only used 61000-4-3 to demonstrate the fallacy of worrying about such small deviations when the overall requirement in a really well-designed and expensive chamber is +6, - 0 dB. Consider the type of absorber used, the limited frequency range (80 1000 MHz), the total lack of conducting material, and the fact that 25% of the UFA may be excluded from the 6 dB variation.
Your variations are well within that tolerance, and your room and set-up nowhere near that good. If you want better than what you have, you will need a chamber and set-up much better than that for 61000-4-3 impractical, to say the least. The point is, make sure you have enough amplifier to do the job with some margin. That¹s as good as you are going to get. Ken Javor Phone: (256) 650-5261 From: John Woodgate <[email protected]> Reply-To: John Woodgate <[email protected]> Date: Mon, 5 Mar 2018 16:19:10 +0000 To: <[email protected]> Subject: Re: [PSES] Field probe calibration +/- 20% doesn't seem to be enough to explain the reported result. After all, assuming the +/- 20% is off the spectrum analyser, 1.2 is +1.6 dB and 0.8 is -1.9 dB. These are small, but not negligible. If the +/-20% relates to power, they are even smaller in dB, of course. John Woodgate OOO-Own Opinions Only J M Woodgate and Associates www.woodjohn.uk <http://www.woodjohn.uk> Rayleigh, Essex UK On 2018-03-05 15:57, Patrick wrote: > > Hi David - > > > > I had this same problem. > > I'm actually glad to see another engineer looking into this! > > The answer is not test setup, > > > not ground plane, > > not distance to tabletop, etc. > > > Each of those can be significant, and should be controlled, but... > > > > > > > > > The problem you describe sounds exactly like one I had two years ago. > > If it is the same problem, the root cause is the *probe calibration*. > > The "normal" calibration data that every cal lab provides is too course. > > It hides (skips over) resonances in the probes. > > Only a finer calibration step size will resolve this problem. > > > > > > > > Here is my abbreviated story... > > > > > > I started with data just like yours. > > I had 6 probes: they were two model FP4000, and four of the HI6053. > > > > > ... <skip the description of months long investigation of setup, antennas, > amplifiers, chambers, height above table, orientation, etc....> > > > > > The root cause is the "normal" cal. > > It uses a course step size in frequency. > > When it is too course, it misses resonances in the probe. > > Those resonances are significant, as much as +/- 20% in my equipment. > > > > > To prove this, I sent a probe out for "enhanced" cal. > > I requested both a "normal" cal and a higher resolution cal. > > I asked for 5% steps below 1 GHz and 100 MHz steps above 1 GHz. > > > > > When the data came back I plotted the cal factors on top of each other. > > It was obvious. > > The course cal points of a "normal" calibration will hide resonances that are > +/- 20% deviations. > > > (above sentence should be BOLD, UNDERLINE, asterisks) > > > > > > > > My conclusion: Any probe used for accredited test must have calibration data > showing the resonances. > > If it doesn't, then the lab is guaranteed to be over-testing and > under-testing. > > > > > DM me and I'll be glad to discuss. > > > > > Patrick > > > > > > > > > > > On Sun, Mar 4, 2018 at 8:33 PM, Schaefer, David <[email protected]> wrote: > >> Ken, >> >> This data was not taken with 61000-4-3 primarily in mind. We do -4-3, but >> also MIL, RTCA, and ISO testing. I should have had the probe at least 15 cm >> for ISO or 30 cm for MIL like you said, but 10cm is how I took the data. >> >> Uniform field calibrations will be a concern eventually, but the variance is >> my problem. This was not four probes set up on a bench next to each other. >> This was data with one probe on the bench, centered in front of the antenna, >> then removed and replaced as precisely as possible with the next probe. >> >> So if I do a single point cal for ISO 11452-2, one probe might tell me 100 >> V/m and another 140 V/m. I'll get questioned by customers if they fail one >> day and pass another. This also runs into another issue - purchasing >> amplifiers. If I specify an amp to reach a desired field strength but when it >> shows up we can't hit levels due to using a different field probe, there will >> be hell to pay. >> >> Standards are silent on probe orientation as well. Do you position the >> probe to maximize field strength? If I can get an extra half a dB of power by >> having it an angle instead of straight on, why not do that? I can save that >> amplifier cost - at least until I get a new probe. The calibrations don't >> seem to mean that much based on my data, so with a composite reading >> whichever probe orientation gives me the highest field should be ok. >> >> Also, any replies I make may be delayed. It seems like I usually see a 4+ >> hour delay between when I email the listserve, and when it is delivered. >> >> Thanks, >> >> David Schaefer >> >> >> From: Ken Javor [mailto:[email protected]] >> Sent: Sunday, March 04, 2018 12:17 PM >> To: [email protected] >> Subject: Re: [PSES] Field probe calibration >> >> In turn: >> >> It is not surprising at all that it takes less power to generate the >> vertical field than the horizontal field. That's the effect of the >> conducting ground plane. The OP doesn't say what spec they are working to, >> but that is why MIL-STD-461 below 1 GHz has the probe 30 cm above the ground >> plane, to limit that effect. >> >> Comments, such as Gert Gremmen's, that measurements in the presence of a >> ground plane (or any conducting structure) are useless, are themselves >> useless. The comment reflects a difference in standards of value. If one is >> starving, food is the most important priority. If one is asphyxiating, oxygen >> is the primary need. It is logically incorrect for two people suffering >> these two conditions to point at each other and say the other one is wrong >> about his priorities: they are both correct within the scope of their >> individual circumstances. The only logical observation that can be made is >> that oxygen needs to be supplied sooner than food, if the standard of value >> is immediate survival. >> >> In the world of goods slated for use in home, office and factory, the coin >> of the realm is accuracy and minimum uncertainty, so that qualifications >> everywhere result in a level economic playing field. Required field >> intensities (1/3/10 V/m) are very low compared to the world of vehicle EMI >> testing (as high as 200 V/m, sometimes beyond), so that (again relative) low >> power amplifiers may be used with antennas separated from the test area by >> three meters instead of one, facilitating the calibration of a quiet zone in >> the complete and utter absence of any conducting surfaces, because the >> end-item use does not include installation on or near a conducting ground >> plane. >> >> This is all in sharp contrast to the qualification of equipment slated for >> use on a metal or partially metallic vehicle. The ground plane is of >> material use in reducing the intensity of the horizontal field near it, as >> noted in the OP and taken advantage of by the very standards that deal with >> such qualifications: the ground plane is our friend. Let us count the ways: >> >> As mentioned above, the ground plane reduces the intensity of a horizontally >> polarized field in its immediate vicinity. >> >> The presence of a ground plane causes cables in its vicinity to react to >> fields not as an antenna as in the 61000-4-3 and 61000-4-6 paradigm, but as a >> transmission line. Transmission lines radiate less per unit of current they >> carry, and couple less power from an incident field, than do antennas. Given >> the very stringent RE and RS requirements in vehicle standards, we need all >> the help we can get. >> >> And finally, metallic equipment enclosures bonded to a ground plane allows >> filters to efficiently shunt incoming noise to the ground plane and away from >> internal circuitry, and perform that same function for noise currents coupled >> to shields that terminate in a low impedance manner to the exterior of such >> metallic equipment enclosures. >> >> Now having dealt with Gert - his recurring comment about the futility of >> vehicle EMI testing re ground planes consistently eliciting the above >> response from me, ad infinitum and ad nauseum, lets look at the OP provided >> test data, especially in light of the 61000-4-3 required UFA (uniform field >> area) uniformity requirement of +6 dB, -0 dB for 75% of the required sixteen >> measurement points. >> >> What I see is that for a given polarization and frequency every single >> position measured is within 6 dB of the others. In the immediate presence of >> a ground plane, no less! The cup is not half full - the cornucopia is >> overflowing. This performance greatly exceeds the MIL-STD-461 requirement: >> there is no requirement for multiple measurement points, and if such are >> used, the only requirement is to use the average of the measured points as >> the leveling field intensity. In other such standards, such as RTCA/DO-160, >> there is a requirement to precalibrate the field in the absence of the test >> sample; I would say that the OP test data is evidence of an excellent >> chamber. >> >> If I wanted minimal variation between various positions down the length of a >> ground plane, I would not use an aperture antenna such as a DRG horn, but >> rather one with constant beamwidth vs. frequency, such as a log periodic. >> Assuming I could get the required field intensity with the amplifiers at >> hand. >> >> Finally, while all the standards of which I am aware allow leveling on the >> composite output of field probes looking in three orthogonal direction at >> once, it is unsurprising that this results in significant variations in >> required power level. Better testing results when the test equipment allows >> leveling on the polarization of interest. Although, as I said above, I >> consider the cited test data below to be exemplary. >> >> I would have been bragging to my colleagues, not complaining! >> >> Ken Javor >> Phone: (256) 650-5261 <tel:%28256%29%20650-5261> >> >> ________________________________ >> From: Cortland Richmond <[email protected]<mailto:[email protected]>> >> Reply-To: Cortland Richmond <[email protected]<mailto:[email protected]>> >> Date: Sun, 4 Mar 2018 08:01:31 -0500 >> To: <[email protected]<mailto:[email protected]>> >> Subject: Re: [PSES] Field probe calibration >> >> >> >> I'm with Gert. >> >> >> Anything "antennas" is checked in the far-field -- especially if testing >> for accuracy. >> >> I'm a BIG fan of near-field probing for relative measurements and >> localizing emissions, but we use probes appropriate to what we are looking >> for; if I wanted to "calibrate" one there, I'd use a known current on a >> wire/trace or a known voltage on a small plate -- and not trust *that* much. >> >> >> >> >> >> >> >> Cortland Richmond >> >> >> >> On 3/4/18 5:35 AM, Gert Gremmen; ce-test wrote: >> >> >> >> >> IMHO all probes are calibrated under far field conditions. >> >> >> In general: Using probes in the proximity (< lambda) of anything conductive >> (including ground planes at 10 cm and including EUT) makes the measurement >> data useless. >> >> >> >> As James correctly states, the construction of the probe makes this effect >> different per type of probe, be it the construction, the size of battery or >> electronics on board or the lead (fiber or copper) , as long a other >> conductors are in proximity the read out has no relation to calibration data >> anymore. >> >> >> Using a probe near a ground plane, such as usual in automotive test set ups, >> indeed says not much about the test level of the EUT. >> >> >> >> >> >> >> Repeating this test under far field conditions, preferable on an antenna >> calibration facility, might give you much better results. (not that you are >> allowed to generate this much of power on air ;<) >> >> >> >> Gert Gremmen >> >> >> >> On 4-3-2018 11:06, James Pawson (U3C) wrote: >> >> >> >> >> >> Hi David, >> >> >> >> An interesting set of results! I'm going to ask some questions that I'm sure >> you've already considered so please bear with me being Captain B. Obvious. >> >> >> >> Do your field probes use frequency correction? I'm not familiar with a wide >> range of probes but my Narda PMM field probe has an internal calibration >> table; you tell it what the field frequency you are applying is and it makes >> the appropriate correction. However, looking at the typical correction data >> from the manual (see PDF page 12 of this doc: >> https://www.emctest.it/public/pages/strumentazione/elenco/Narda/EP%20600/Manu >> ali/EP600-EP601EN-90302-2.02.pdf) it doesn't look like a large difference. >> >> >> >> Is there a difference in the probe construction between the probes used? >> Some probes like the Narda one above have two antenna per axis whereas ones >> like this Amplifier Research probe - >> https://www.arworld.us/html/18200.asp?id=636 only have one antenna per axis. >> Perhaps the proximity of copper plate makes a difference. >> >> >> >> On the subject of copper plate, what are the differences without this >> present? What are the dimensions of it and are they significant at the >> frequencies selected? >> >> >> >> Have you acquired just spot readings or a full frequency sweep? There may be >> some patterns in the frequency sweep data that give you more of a clue as to >> what's happening. >> >> >> >> An interesting puzzle and I look forward to hearing about your results >> further! >> >> All the best >> >> James >> >> >> >> >> >> >> >> >> >> >> From: Schaefer, David [mailto:[email protected]] >> >> - >> ---------------------------------------------------------------- >> This message is from the IEEE Product Safety Engineering Society emc-pstc >> discussion list. To post a message to the list, send your e-mail to >> <[email protected]> >> >> All emc-pstc postings are archived and searchable on the web at: >> http://www.ieee-pses.org/emc-pstc.html >> >> Attachments are not permitted but the IEEE PSES Online Communities site at >> http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used >> formats), large files, etc. >> >> Website: http://www.ieee-pses.org/ >> Instructions: http://www.ieee-pses.org/list.html (including how to >> unsubscribe) >> >> >> List rules: http://www.ieee-pses.org/listrules.html >> >> For help, send mail to the list administrators: >> Scott Douglas <[email protected]> >> Mike Cantwell <[email protected]> >> >> For policy questions, send mail to: >> Jim Bacher: <[email protected]> >> David Heald: <[email protected]> >> >> > > > > > > > -- > > > // > Patrick > > > > - > ---------------------------------------------------------------- > > > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. To post a message to the list, send your e-mail to > <[email protected]> > > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieee-pses.org/emc-pstc.html > > > Attachments are not permitted but the IEEE PSES Online Communities site at > http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used > formats), large files, etc. > > > Website: http://www.ieee-pses.org/ > Instructions: http://www.ieee-pses.org/list.html (including how to > unsubscribe) <http://www.ieee-pses.org/list.html> > List rules: http://www.ieee-pses.org/listrules.html > > > For help, send mail to the list administrators: > Scott Douglas <[email protected]> > Mike Cantwell <[email protected]> > > > For policy questions, send mail to: > Jim Bacher <[email protected]> > David Heald <[email protected]> > - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) <http://www.ieee-pses.org/list.html> List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
